The present invention is directed to monitoring for the presence of frequency-modulated signals such as those produced by chirp radars.
In a chirp radar, a radar transmitter transmits a carrier whose frequency is swept through a range of frequencies and whose amplitude is modulated by pulses that typically are smooth and of relatively long duration. A common method of producing this type of signal is to generate a short-duration baseband pulse, band-limit it, typically with a Gaussian filter to provide a short-duration oscillatory signal, and apply the oscillatory signal to a dispersive delay line, which delays different frequencies by different amounts and thus spreads the signal in time. The spreading of the signal results in lower instantaneous power for a given average power. Despite the relatively long duration of the dispersed signal, however, a range resolution can be achieved that is approximately the same as that possible with the undispersed pulse. This is accomplished by using a reverse of the dispersive delay on reception to recompress the returned pulses.
In attempting to detect the presence of such radars and distinguish one from another, it is usually desirable to determine the chirp rate, or time rate of change of frequency, of the chirp radar as rapidly as possible. It has previously been proposed to analyze such signals by observing the frequency spectrum that results when they are applied to a compressive receiver, a device that responds to an input by generating an output whose time of occurrence depends on the frequency of the input. The output of a compressive receiver in response to a narrow-band signal is a short burst of the compressive-receiver center frequency, whereas the output in response to a chirp signal is a longer-duration burst. However, although the power spectrum of the output that results from a chirp signal can in some cases differ markedly from that resulting from a narrow-band signal, the difference is often minimal. Thus, it is often difficult by that method even to distinguish between chirp signals and narrow-band signals, and it is even more difficult to distinguish between chirp signals of different chirp rates. It is accordingly an object of the present invention to determine chirp rate automatically and in a reliable manner.